1. Field of the Invention
This invention relates generally to a variable reactance circuit, and is directed more particularly to a variable reactance circuit which is adapted for use in, for example, a variable frequency oscillator, as FM modulator and the like.
2. Description of the Prior Art
In the prior art, there has been proposed a variable reactance circuit employing a varactor diode. However, such variable reactance circuit has a narrow range or variation and is not preferred for formation as an integrated circuit.
It has also been considered to provide a variable capacitive reactance circuit which is composed of a series circuit of a constant current source and a diode, a transistor with its emitter electrode grounded, a control voltage source connected between the base electrode of the transistor and the anode electrode of the diode, and a capacitor connected between the collector electrode of the transistor and the anode of the diode. However, with this type of the variable capacitive reactance circuit, if its output impedance is required to be great, an inductance element is necessary and the capacitance of the capacitor must be made great so as to increase the range of variation of the reactance. The inclusion of an inductance element and/or the need for a capacitor having a very large capacity make it difficult to form this variable capacitive reactance circuit as an integrated circuit.